In automation technology, position-measuring devices are used to determine, in closed-loop drive mechanisms, instantaneous position values which are needed by subsequent electronics, such as a numerical control system, to calculate setpoints for control circuits used to control the drive mechanism (e.g., the feed rate of a tool or workpiece). If the position-measuring devices are in the form of rotary encoders or angle-measuring devices, then these are directly or indirectly coupled, for example, to the shaft of a motor for this purpose. Length-measuring devices measure, for example, linear movements between a machine bed and a machine part that is positionable relative to the machine bed, such as, for example, a movable tool carriage.
Today, absolute position-measuring devices are preferably used. Such devices generate absolute measurement values, which are transmitted from the position-measuring device to the subsequent electronics via digital, mostly serial data interfaces. The measurement values are mostly position values (angular values or linear positions), but there are also known position-measuring devices which deliver velocity or acceleration values; i.e., measurement values which indicate changes in positions over time.
It is often necessary to be able to trigger events in the position-measuring device from the subsequent electronics via the serial data interface. This applies especially to measurement value requests.
For example, European Patent Application EP 0 660 209 A1 describes a position-measuring device having a synchronous serial interface; i.e., an interface where the time sequence of the data transmission on a (bidirectional) data line is controlled by a clock signal which is fed to the position-measuring device via a separate clock line. In this position-measuring device, the acquisition of the samples, which are subsequently processed into a position value, is triggered by the first edge of the clock signal. This first edge at the same time signals the beginning of a data transmission. Thus, the first edge of the clock signal serves as a trigger signal for the acquisition of the samples, and thus triggers the generation of a measurement value.
This approach cannot be used for serial interfaces that do not have a clock line. Moreover, in position-measuring devices having very fast signal-processing electronics, the measurement value (position values) may happen to be available at a time somewhat before it can be transmitted to the subsequent electronics in accordance with the rules of the interface protocol. Thus, from a control engineering point of view, this results in additional dead time (i.e., the measurement value is “outdated”). This means that when the measurement value arrives at the subsequent electronics, the drive mechanism has already moved further by a certain distance or through a certain angle. This problem can be minimized by mathematical algorithms (extrapolation), which function all the better, the more precisely the point in time of sample acquisition in the position-measuring device is known and the shorter the period of time is between the point in time of sample acquisition and the arrival of the measurement value at the subsequent electronics.